The present invention relates to a connection for joining pipes together, in particular to pipes for use underground in, for example, the gas and oil industry.
When transporting a fluid, such as oil or natural gas between two locations, the most cost effective methods, where possible utilise pipelines. Such pipelines are normally constructed from individual pipes joined together. The regions where the pipes join, constitute discontinuities in both the inner and outer surface of the pipeline and as such are sources of weakness within the finished pipeline.
It is important that any such weakness is minimised as much as possible, as any loss of fluid from the pipe is both wasteful of resources and potentially disastrous for the environment. Furthermore, the fluid lost can also increase the risk to personnel from fire or explosion. Replacement of inefficient or leaking joints is not always easy. Particularly in the case of the oil and gas industry, pipelines are situated quite often in relatively inaccessible locations and replacement is difficult and expensive.
A number of solutions have been employed to overcome weakness. The most common solution is a screw-threaded joint, the threads of the joint acting together to prevent separation of the individual pipes either axially or radially.
The joints formed must firstly be so constructed that any fluids within the pipe cannot escape. They must also be capable of withstanding large differentials in pressure across the radial width of the joint, which pressure differential usually has a positive component in an outwardly radial direction.
Screw-threaded joints require the engagement of two threaded regions situated one on each of the pipes to be connected together. Engagement is normally by application of a torque. The two threaded regions are normally manufactured so that their threads are complementary to each other.
GB 1 587 836 (VALLOUREC) discloses a screw connection for pipes in which a stop-shoulder on one pipe co-operates with a complementary stop-shoulder on the other pipe to form a seal.
There a number of problems with such a seal. Firstly, the metal from which the stop-shoulders are formed, flows under the high torque applied when screwing the pipes together. Therefore if care is not taken slippage or buckling of the joint can occur. Secondly, the joints formed can comprise a step discontinuity in going from one section of pipe to the other.
It is an object of the present invention to provide a screw-thread connection which addresses the above problems
According to the present invention there is a provided a screw threaded joint for pipes comprising a first pipe length having at one end a male screw threaded portion and a second pipe length having at one end a female portion having a complementary screw thread, the portions being adapted to inter-engage alone the greater part of the axial length of the threaded portions, the screw threads thereof being inclined in the same direction and at an acute angle to the longitudinal axis of the pipe length, at least the male thread extending to a position adjacent a complementary stop shoulder on the other portion, characterised in that the or each complementary stop shoulder comprises a recess in the form of a cone having a rounded apex, the recess being adapted to sealingly receive a head in the form of a cone having a rounded apex with a radius marginally greater than the radius of the recess thereby providing a sealing surface with improved stress dispersal properties upon application of a torque to the joint.
The wall of the cone on the in-use outer surface of the male screw-threaded portion preferably subtends an angle of from 28 to 36xc2x0 to the longitudinal axis of the pipe. Particularly preferably the wall of the cone subtends an angle of from 30 to 33xc2x0 to the longitudinal axis of the pipe, and most preferably an angle of 30xc2x0.
The corresponding cone wall on the female threaded portion preferably subtends an ankle of from 28-38xc2x0 with the longitudinal axis of the male threaded portion. The angle subtended by the cone wall of the female threaded portion is preferably greater than that subtended by the cone wall of the male threaded portion.
The male screw threaded portion preferably comprises a stress dispersing surface connecting the stop-shoulder to the first thread of the pin. The stress dispersing surface preferably describes a concave curve.
The radius of curvature of the concave curve of the stress dispersing surface is preferably from 0.075 to 0.085 cm, and is most preferably 0.081 cm.
The male screw threaded portion preferably comprises an end-shoulder which subtends an angle of from 14-24xc2x0 with the plane perpendicular to the longitudinal axis of the pipe. The end-shoulder preferably subtends an angle of from 18-20xc2x0. It particularly preferably subtends an angle of 20xc2x0.
The end-shoulder of the female threaded portion preferably subtends an angle of 12-24xc2x0. With the plane perpendicular to the longitudinal axis of the male threaded portion. The angle subtended is conveniently from 14-18xc2x0. The angle subtended by the end-shoulder of the female threaded portion is preferably less than that subtended by the end-shoulder of the male threaded portion.
When the connection is axially compressed, the male screw threaded portion has a tendency to bulge inwards due to inwardly directed radial forces generated by the compression. The deformation thus caused is often known as barrel deformation.
Advantageously the in-use inner surface of the stop-shoulder is preferably concave to reduce the effects of barrel deformation. The radius of the concave surface is preferably from 4.9 to 5.2 cm, and most preferably 5.1 cm.